Field of the Invention
The present invention relates to a magnetic resonance measurement apparatus, and in particular to a technique for sampling a reception signal.
Description of Related Art
As magnetic resonance measurement apparatuses, nuclear magnetic resonance (NMR) measurement apparatuses and electron spin resonance (ESR) measurement apparatuses are known. In addition, as apparatuses classified as NMR measurement apparatuses, magnetic resonance imaging (MRI) apparatuses are also known. In the following, NMR measurement apparatuses will be described.
NMR refers to a phenomenon where an atomic nucleus under a static magnetic field interacts with an electromagnetic wave having a frequency intrinsic to the atomic nucleus. An apparatus that executes measurement of a sample at an atomic level taking advantage of this phenomenon is an NMR measurement apparatus. Currently, NMR measurement apparatuses are used in analyses of organic compounds (for example, medicines and agricultural chemicals), polymer materials (for example, vinyl and polyethylene), biological substances (for example, nucleic acids and proteins), or the like. With the use of an NMR measurement apparatus, for example, a molecular structure of the sample can be revealed.
An NMR measurement apparatus generally includes a control computer, a radio frequency (RF) signal transmitter, an NMR signal detector (probe), a static magnetic field generator (superconductive magnet), an NMR signal receiver, and the like. In some cases, a part of these structures is called an NMR measurement apparatus. For example, a part of a spectrometer including the control computer, the RF signal transmitter, and the NMR signal receiver may be called an NMR measurement apparatus. In a typical NMR measurement, a high-frequency signal for NMR measurement (RF transmission signal) is generated in the transmitter, and the transmission signal is supplied to a transmission and reception coil in the probe. A resonance absorption phenomenon is caused in an observation nucleus in the sample due to an electromagnetic wave caused by the transmission signal. An NMR signal induced in the transmission and reception coil (RF reception signal) is then transmitted to the receiver, and a spectrum of the received signal is analyzed.
In general, in sampling of the reception signal, the sampling must be executed at a frequency of greater than or equal to twice the frequency of the reception signal (Nyquist's theorem). In an NMR measurement apparatus, if the sampling is to be executed by an ADC (analog-to-digital (A/D) converter) for a reception signal of a few hundred of MHz, the sampling must be executed at a very high frequency. Such an A/D converter that operates at a high frequency is in general difficult to obtain, and, even if such an A/D converter is available, the A/D converter would be very expensive, and in many cases, does not have a sufficient resolution. On the other hand, the observation frequency is known in the NMR measurement. Under such circumstances, in some of the NMR measurement apparatuses, an under-sampling technique is utilized (refer to JP H6-98874 A, JP 2010-139316 A, and JP 2011-102804 A). In these techniques, sampling is executed at a frequency lower than twice the frequency of the signal to be processed (in particular, at a lower frequency than the frequency of the signal to be processed), and a mirror signal component which is caused by a target signal component being aliased or folded is observed. For example, in the NMR measurement apparatus of related art disclosed in JP H6-98874 A, an eighth-order aliased signal component is observed. On the other hand, when the transmission and reception are executed at a relatively low RF frequency (for example, when the RF frequency is less than half of the sampling frequency), over-sampling is possible, and is appropriate.
In the NMR measurement apparatuses of related art disclosed in JP H6-98874 A, JP 2010-139316 A, and JP 2011-102804 A, the over-sampling and under-sampling are not switched. Thus, it has not been possible to selectively use an appropriate sampling scheme according to the situation. This problem may also occur in magnetic resonance measurement apparatuses other than the NMR measurement apparatuses.